Exhaust upgrades are without a doubt the most popular upgrade for any performance vehicle, and more often than not, are the very first area that any new car buyer looks to when the itch for modification takes it’s grip. Exhaust systems improve a vehicle’s performance, efficiency, and most importantly, provide a great new sound that truly unleashes the audible assault on the senses constricted by the factory exhaust. The 2011 Ford Mustang, with its 5.0-liter engine, is already one of the most popular Mustangs ever produced, and as such, aftermarket performance parts, including mufflers and exhaust systems, are very much in high demand. Flowmaster, one of the leading manufacturers of exhaust systems and components for all things automotive, is right there with a response to the masses, releasing two new Axle-Back exhaust systems for the 2011 Mustang GT that offer two differing sound levels and tones.

“In creating these kits for the 2011 Mustang, we essentially took the challenge,” said Cam Benty, Flowmaster’s VP of Marketing. “We ran into a situation where we were told by the Ford engineers that there was just no way to make a better system and increase horsepower and torque. Well, we’ve been able to do both, and these programs have worked out extremely well. They both feature similar designs, but obviously are modified in such a way to adapt to the Flowmaster or Hushpower mufflers.”

The American Thunder series chambered muffler delivers that aggressive, performance-minded tone from the Mustang that blue oval enthusiasts have come to know and love over the years.

The American Thunder Series of mufflers have long been a staple of Flowmaster’s acclaimed lineup, and the 2011 Mustang offering is the most recent addition to the mix. If you want your presence known, the American Thunder Series is just the ticket, and this one certainly continues that legacy. This exhaust kit is aimed at the serious enthusiast who desires all the performance benefits that an axle-back dual exhaust system offers, delivering a moderate to aggressive tone. With maximum performance and easy fit in mind, the system includes mandrel bent tubing and utilizes mufflers with Flowmaster’s Delta Flow technology. Made of stainless steel, the kit has a 3-inch inlet and outlet diameter.

The Hushpower muffler for the 2011 Mustang is much different design from the Flowmaster American Thunder, featuring a more straight-through directional system with some unique technology utilizing a series of cones and proprietary insulation material that keeps the heat away from the bottom of the car while also delivering a very different but still aggressive tone.

Delivering that signature Flowmaster sound with an overall more moderate tone, the Hushpower Pro Series Axle-Back system delivers dyno-tested horsepower and torque improvements for the 5.0-liter Mustang. Also available in cat-back form, it sports a full 3-inch, mandrel bent stainless steel construction with a Scavenger X-pipe assembly for mid-range torque improvement and a pair of Hushpower Pro Series mufflers that deliver a traditional, alternative sound to the Flowmaster Chambered design like that of the American Thunder.

The dual exhaust system is finished off with a pair of 4-inch polished tips that exit through the factory cut outs. And like all Flowmaster exhaust systems, the benefits include improved throttle response, added horsepower, and increased fuel economy. Dyno testing has shown an average performance gain of 8 horsepower and 12 ft-lbs of torque with both of these systems on the 5.0-liter Mustang.

“The systems themselves are very similar in terms of pipe design because you’re confined by the bottom of the car and that’s going to orchestrate some of the design, so it’s really the mufflers that are different. We’re using 3-inch pipe and obviously thats key because we want to get as much flow through as possible and have the best position for our mufflers,” explained Benty.

Installing the Flowmaster Axle-Backs

The axle-back section of the exhaust system is coupled via an AN-type fitting that requires no cutting or welding, making installation a snap. This fitting also allows for some adjustment once the Flowmaster system is in place for proper positioning. The clamp is held to the stock muffler with a spot weld, though it can easily be removed.

Both systems offer a true bolt-on installation with no cutting or welding required, making the swap from the factory exhaust one the enthusiast could complete in an hour or two. To begin the process, we must first remove the factory exhaust components.

We begin by removing the pipe coupling located just behind the axle housing, and then unbolting the bracket that holds the two rear hangars. There are three bolts on the driver’s side hangar and two on the passengers. All the bolts on the brackets are 13mm in size.  We can then slide that bracket towards the middle of the car, which will release the front hangar from its mounting location and the entire assembly will drop out.  If you are having difficulty sliding the muffler off the front hanger, there are two bolts that are easy to get to that will drop out the hangar.

Then we get to the fun part of installing the Flowmaster goods. The aftermarket muffler and tip assembly fits nicely back into place, utilizing the factory exhaust hangar bracket and locations. Once the hangars are in place and the pipes have been re-coupled using the AN-type fitting, and the position of the mufflers, pipes, and tips have been adjusted for a satisfactory fit, we’re ready to hit the ground running.

Both the Flowmaster American Thunder and Hushpower axle-back systems fit directly up to the factory exhaust hangars and brackets. There are three bolts on the driver's side rear hanger and two on the passenger. All are 13mm in size.

Testing The Exhausts

For our road testing, we put our Mustang equipped with both the American Thunder and Hushpower Axle-Back exhaust systems through a series of driving situations to properly evaluate the sound levels produced by both systems. In doing so, we compared the interior and exterior resonance levels found in each kit at idle, revving, cruising speed drive-by, and launching with the hammer down where aftermarket exhaust kits really sing a tune.

The American Thunder Axle-Back system, as advertised, delivers a level of sound that is sure to get attention. While producing a moderate rumble at idle, this system comes alive when given a healthy amount of throttle with deep, rich, and aggressive tones throughout the spectrum. Inside, this system and the Hushpower are very comparable, although the upper hand in decibel levels goes to the Hushpower. Said Benty, “The chambered Flowmaster muffler provides that signature Mustang sound that we all grew to know and love in the 90’s and it continues on with American Thunder system.”

Both of these high performance axle-back exhaust kits sport 4" polished stainless steel exhaust tips, giving your Mustang a great look and alerting others that you're packing Flowmaster's.

The Hushpower system, meanwhile, has a pretty aggressive tone as well, producing sound levels in line with the American Thunder system but with a deep, throaty exterior tone that differs greatly. Inside, this system provides a moderate tone at cruising speeds, but becomes rather aggressive at wide-open throttle. “The Hushpower has a little deeper and throatier sound with just an all-around different style, but very performance-oriented,” says Benty.

Benty continued, “Both systems incorporate elements that are very effective and very efficient, and both will improve horsepower and torque. The real difference in these two systems is basically the tone. Some people like the Flowmaster and some like the Hushpower: it’s a matter of personal preference. You can go on our website and actually hear the systems in place and really get a read on which system is best for you. Bottom line, they both increase horsepower and torque; very cool systems with a very good fit and dressed out with a polished stainless tip to let everyone know you’re packing Flowmasters.

Despite being told otherwise by Ford engineers, Flowmaster has managed to increase both horsepower and torque figures with both of their axle-back exhaust systems while delivering that distinct tone they've made so famous over the years.

Regardless of which direction that you decide to take for the exhaust upgrade on your 2011 Mustang, you can rest assured that you’re getting that famous sound and quality that only a Flowmaster exhaust system can provide. And while adding a whole new upgraded element to your own driving experience, you’re sure to get attention while improving your performance, efficiency, and fuel mileage all at the same time.